Many images online are in public domain and can be used on your website.
However, you should avoid using images with water marks on them.
That being said, criminals are taking full advantage of your fear of copyright infringements.
Copyright Scams
When you receive an email demanding money, start by considering a scammer trying to get you to pay them.
If the work was truly a copyright matter, the information below would be needed to be sent to you.
1. The name(s) of the author(s) of the work the date and location of authorship.
2. The name of the owner of the work and the address of the owner of the work.
When you get an email regarding copyright matters, ask them to provide;
1. All documents filed with the copyright office.
2. The authority, issued by the government, to claim copyrights.
You will find that your copyright matter will disappear into the dark.